Routing devices within a network, often referred to as routers, maintain routing information that describe available routes through the network. Upon receiving an incoming packet, the routers examine information within the packet and forward the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more defined routing protocols, such as the Border Gateway Protocol (BGP).
Multi-protocol Label Switching (MPLS) is a mechanism used to engineer traffic patterns within Internet Protocol (IP) networks. By using MPLS, a source device can request a path through a network, i.e., a Label Switched Path (LSP). An LSP defines a distinct path through the network to carry MPLS packets from the source device to a destination device. A short label associated with a particular LSP is affixed to packets that travel through the network via the LSP. Routers along the path cooperatively perform MPLS operations to forward the MPLS packets along the established path. LSPs may be used for a variety of traffic engineering purposes including bandwidth management and quality of service (QoS). A packet may be a formatted set of data.
A variety of protocols exist for establishing LSPs. For example, one such protocol is the label distribution protocol (LDP). Another such protocol is the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE). RSVP-TE uses constraint information, such as bandwidth availability, to compute paths and establish LSPs along the paths within a network. RSVP-TE may use bandwidth availability information accumulated by a link-state interior routing protocol, such as the Intermediate System-Intermediate System (ISIS) protocol or the Open Shortest Path First (OSPF) protocol.
Head-end routers of an LSP are commonly known as ingress routers, while routers at the tail-end of the LSP are commonly known as egress routers. Ingress and egress routers, as well as intermediate routers along the LSP that support MPLS, are referred to generally as label switching routers (LSRs). A set of packets to be forwarded along the LSP is referred to as a forwarding equivalence class (FEC). In general, each router along the LSP maintains a context that associates a FEC with an incoming label and an outgoing label. This association is referred to as a FEC-label binding. The ingress label edge router (LER) uses routing information, propagated from the egress LER, to determine the LSP, to assign labels for the LSP, and to affix a label to each packet of the FEC. The LSRs use MPLS protocols to receive MPLS label mappings from downstream LSRs and to advertise MPLS label mappings to upstream LSRs. When an LSR receives an MPLS packet from an upstream router, the LSR performs a lookup in the context and swaps the MPLS label according to the information in its forwarding table based on the lookup and forwards the packet to the appropriate downstream LSR or LER. The egress LER removes the label from the packet and forwards the packet to its destination in accordance with non-label based packet forwarding techniques.
The LSRs may use LDP to distribute labels to other routers along hop-by-hop paths to establish LSPs. More specifically, the routers may distribute LDP label mapping messages indicating assigned labels and associated FECs for the LSPs. Two routers that use LDP to exchange label mapping information are referred to as LDP peer routers. Directly connected neighbor routers may be LDP peer routers with a direct LDP session. In the case where non-neighbor routers want to be LDP peer routers, the non-neighbor routers may establish a targeted LDP session between them. More information on LDP and targeted LDP sessions is described in Anderson, L., “LDP Specification,” RFC 5036, October 2007, the entire contents of which are incorporated by reference herein. More information on MPLS is described in Rosen, E., “Multiprotocol Label Switching Architecture,” RFC 3031, January 2001, the entire contents of which are incorporated by reference herein.